A major application of high field ENDOR consists in performing hyperfine measurements at different field positions in the EPR line corresponding to the excitation of highly selected molecular orientations. Since the g-anisotropy dispersion scales with the external magnetic field, the 140 GHz ENDOR spectra reveal field dependent powder patterns that provide a large number of constraints for the determination of the hyperfine couplings. We used this technique to investigate the electronic and chemical structure of the tyrosyl radical in different Class I RNRs, such as from bacterium E. coli and Yeast. Magnetic field dependent ENDOR spectra were obtained with high signal-to-noise (> 20) and revealed essential new features not discerned in X-band experiments. A simulation program was developed in order to extract hyperfine tensors and structural informations reflected in the relative orientation of the hyperfine tensor to the electronZeeman g-tensor. From the analysis of the experimental and simulated spectra of the E. coli tyrosyl radical, we obtained a new assignment for the hyperfine couplings of the weakly coupled 2,6-ring and ~methylene protons. Comparison of ENDOR spectra of a RNR enzyme in protonated and perdeutated water matrix confirmed the absence of weakly coupled hydrogen-bonded protons. Instead, ENDOR spectra of the Yeast revealed some new features, which we assign to the presence of hydrogen bonds. ENDOR experiments on the Yeast tyrosyl radical are currently in progress.